1. Field of the Invention
The present invention relates to the lubrication of high pressure hydraulic systems, particularly those employed in aircraft by using, as the hydraulic fluid, phosphate ester based fluids.
2. Description of the Related Art
Phosphate ester based hydraulic fluids are well known in the lubrication art and have been utilized for years in hydraulic systems operating at pressures of about 3000 psi and lower.
Phosphate ester based functional fluids useful as aircraft hydraulic fluids have been described in U.S. Pat. Nos. 5,464,551, 3,723,320, 3,679,587.
Aircraft flight control equipment are operated by hydraulic systems. Commercial transport aircraft typically utilize phosphate ester hydraulic fluids while military aircraft utilize mineral oil or polyalphaolefin hydraulic fluids. The standard operating pressure for aircraft hydraulic systems is nominally 3000 psi.
There are a number of examples of aircraft hydraulic systems operating above 3000 psi. Some US military aircraft such as the V-22 Tilt Rotor aircraft have hydraulic systems that operate at 5000 psi. The F/A-18E/F Super Hornet Strike Fighter switches between 3000 psi to 5000 psi operation, depending on aircraft speed, but as previously stated such military aircraft do not use phosphate ester based hydraulic fluids. The Concorde supersonic civil transport aircraft hydraulic system operates at 4000 psi. The Concorde is unusual in that it utilizes a silicate ester hydraulic fluid rather than a phosphate ester hydraulic fluid. There are currently no commercial aircraft using phosphate ester hydraulic fluid which operate above 3000 psi (nominal) hydraulic system pressure.
The commercial aircraft system designers have opted to use phosphate esters because of the superior fire resistance properties of phosphate esters. However, the reduced fire risk is accompanied by a debit in various performance parameters such as lesser stability when compared to the hydrocarbon based fluids used in military aircraft and presence of an unusual corrosion phenomenon. Phosphate esters tend to absorb atmospheric moisture readily and build up high concentrations of water (0.3 to 1.0% water, sometimes more). Like all esters, these fluids have the potential to hydrolyze (react with water to form alcohols and acids). The presence of high concentrations of water typically results in the extent of hydrolysis setting the life of the fluid (point at which the fluid has to be replaced). The corrosion phenomenon peculiar to phosphate esters is termed electro-chemical erosion and it occurs at locations of high fluid velocity and pressure drop. Damage to hydraulic system parts due to electro-chemical erosion will increase rapidly with increases in operating temperature (as would be the case for any corrosion reaction) and with increases in the differential pressure to which a hydraulic pressure device is exposed. The differential pressure mentioned is the pressure difference between hydraulic system main pressure and hydraulic system reservoir pressure (around 50 to 100 psi) to which a hydraulic pressure device is exposed.
Higher hydraulic system operating design pressure would allow a reduction in the size of the hydraulic system components and in the hydraulic fluid inventory of an aircraft hydraulic system. This translates into lower aircraft weight, which is very desirable. A major obstacle to hydraulic system designs of higher than 3000 psi systems for commercial aircraft has been the property limitations of phosphate ester hydraulic fluids. When the Concorde was designed during the late 1960""s, phosphate ester hydraulic fluids were found to be unsuitable and a silicate ester hydraulic fluid was selected for this aircraft. Even though major improvements in the performance of phosphate esters have been achieved since the Concorde experience through the use of potent additives, significant questions remain regarding the ability of phosphate ester aviation hydraulic fluid in operating successfully in a higher than 3000 pressure system, without rapid and significant degradation of the fluid and without damage to hydraulic system components.